1. Field of the Invention
The present invention relates to an image bearing member (also referred to as a photoreceptor or a photoconductor), and an image forming apparatus and process cartridge using the image bearing member.
2. Description of the Background Art
In recent years, organic photoconductors have come to be widely used as image bearing members. Organic photoconductors are advantageous over inorganic image bearing members in terms of ease of developing materials that respond to various kinds of irradiation light sources ranging from visible light to infrared light, selection of materials that are less burdensome on the environment, lower manufacturing costs, etc. By contrast, inorganic image bearing members are superior to organic photoconductors with regard to mechanical durability. In general, image bearing members having an excellent mechanical durability and a long working life are preferable, as is discussed below.
An image forming apparatus employing electrophotography generally includes an image bearing member, a charger that charges the image bearing member, a latent image formation device that forms a latent electrostatic image on the image bearing member with the charger, a development device that attaches toner to the latent electrostatic image on the image bearing member formed by the latent image formation device, a transfer device that transfers the attached toner to a transfer material, a cleaning device that removes residual toner remaining on the surface of the image bearing member, etc.
As such processes are repeated in image formation, the surface of the image bearing member is chemically and/or physically degraded, thereby accelerating abrasion of the organic photoconductor or forming scars thereon. Organic photoconductors are particularly susceptible to such degradation and soon come to produce degraded images. Therefore, mechanical durability is one of the most critical issues confronting development of successful organic photoconductors.
To improve the mechanical durability of organic photoconductors, a number of technologies involving use of a protective layer have been disclosed. In addition, a number of technologies involving improving mechanical durability by dispersing inorganic particulates in the protective layer have been disclosed as well.
For example, Japanese patent application publication no. 2002-139859 (JP-2002-139859-A) describes an image bearing member including an electroconductive substrate on which a photosensitive layer and a protective layer containing a filler are sequentially laminated in that order.
Furthermore, technologies involving increasing the hardness of the surface of the image bearing member are disclosed.
For example, JP-2001-125286-A and 2001-324857-A describe an image bearing member having a reinforced protective layer in order to prevent damage by magnetic particles inadvertently transferred to the image bearing member by a magnetic brush-type charger when the magnetic particles are strongly pressed against transfers and cleaning portions of the image bearing member. In addition, JP-2003-098708-A describes an image bearing member having an enhanced hardness to reduce abrasion of the surface of the image bearing member when a cleaning blade system is used.
As specific measures to enhance the surface hardness of the image bearing member described above, use of cross-linking materials such as thermocuring resins and UV curing resins is known. For example, JP-H05-181299-A, 2002-006526-A, and 2002-082465-A describe use of a thermocuring resin as the binder resin for the protective layer to improve mechanical durability and damage resistance. In addition, JP-2000-284514-A, 2000-284515-A, and 2001-194813-A describe use of siloxane resins combined with a charge transport group in the protective layer to improve mechanical durability and damage resistance.
Furthermore, Japanese patent no. 3194392 (JP-3194392-B) describe an image bearing member having a charge transport layer manufactured using a monomer having a C═C double bonding, a charge transport material having a C═C double bonding, and a binder resin to improve mechanical durability and damage resistance. Similarly, JP-2004-302451-A describes a method of forming a charge transport layer by curing a radical polymerizable monomer having three or more functional groups without a charge transport structure and a radical polymerizable compound having one radical polymerizable monomer with a charge transport structure.
Furthermore, JP-2005-99688-A describes a method of forming a protective layer by curing a radical polymerizable monomer having three or more functional groups without a charge transport structure and a radical polymerizable compound having a charge transport structure and dispersing fillers.
The image bearing members described above have succeeded in achieving a marked increase in mechanical durability. In particular, image bearing members having a protective layer formed of a curable resin like those described in JP-2004-302451-A, JP-2005-99688-A, and JP-2001-166510-A have excellent mechanical durability and damage resistance.
However, it is difficult to make an image bearing member having a long working life by improving just the mechanical durability alone. To further prolong the working life of an image bearing member, prevention of attachment of impurities to the image bearing member and improvement of toner transfer efficiency (toner transfer ratio) are necessary.
That is, even an image bearing member having excellent mechanical durability starts to produce abnormal images once it is used for an extended period of time, because attachment of paper dust and toner additives causes such abnormal images. More specifically, the portions of the image bearing member to which such paper dust and toner additives attach are not properly charged or irradiated, resulting in production of abnormal images.
Since an image bearing member having an inferior mechanical durability is easily abraded, production of abnormal images is consequently limited. However, it is difficult for such an image bearing member to have a long working life.
As for the matter of toner transfer efficiency, as the toner transfer ratio increases, by definition wasteful toner consumption decreases, which is of course desirable in its own right. Moreover, excessive residual toner remaining on the image bearing member after transfer is an added burden on the cleaning device. Consequently, the cleaning effect does not last for a long time, meaning that the life span of a process cartridge is unnecessarily shortened. Thus, it is highly preferable to increase the toner transfer ratio.
Since prevention of attachment of impurities to the image bearing member and improvement of toner transfer ratio have similar characteristics in most cases, both characteristics are referred to as the releasability of the image bearing member. Use of an uppermost surface layer having a low energy is suitable to improve the mechanical durability and impart good releasability.
Such an uppermost surface layer can be obtained by applying a material for reducing the surface energy to the surface of an image bearing member (referred to as external addition) or causing such a material to contain in the layer (referred to as internal addition).
A specific example of external addition is a mechanism of applying zinc stearate, etc. to the surface of the image bearing member. Because of this mechanism, good releasability can be imparted to the surface of the image bearing member. However, material applied to the surface for reducing the surface energy is degraded by repeated discharging, which may cause production of abnormal images. In addition, providing such an application mechanism results in a large-sized image formation portion, thereby imposing restrictions on layout design. Furthermore, the cost of the image formation portion increases.
The internal addition system is also suitable to improve releasability. However, since the surface of the image bearing member is constantly abraded to make the material appear on the surface, these gains in releasability are achieved at the expense of the mechanical durability.
Thus, a good combination of mechanical durability and releasability is difficult to achieve.
JP-2007-178815-A describes an image bearing member using a fluorine-substituted polysiloxane resin for the surface layer to impart a high releasability to the surface of the image bearing member. However, the siloxane bonding is known to cause polarization and form hydrogen-bonding. Therefore, the force of attachment between the polysiloxane resin and toner may increase in a high-humidity environment, with the result that releasability easily deteriorates.
In addition, JP-2002-6526-A describes an image bearing member having a protective layer that contains lubricant particulates. Likewise, JP-2008-139824-A describes an image bearing member having a surface protective layer formed of a cured fluorine-containing curable composition containing a (meth)acrylate having a fluorinated alkyl group and a photopolymerization initiator. Furthermore, JP-2008-233893 describes an image bearing member having a cross-linked type protective layer formed by curing a fluorine-based UV curable hard coating agent and a radical polymerizable compound having one functional group with a charge transport structure and containing lubricative particulates.
Use of a fluorine-based material is suitable to reduce the force of attachment between the image bearing member and toner. In particular, a cross-linked type charge transport layer containing such a fluorine-based material does provide a good combination of mechanical durability and reduction of force of attachment between an image bearing member and toner. However, a considerable amount of fluorine material is required to sufficiently reduce the force of attachment, and since such a fluorine-containing material does not have a charge transport property, addition of a large amount of the fluorine material may lead to an increase of the voltage at a bright portion as well as a tendency for layer strength to decrease.
In addition, JP-2003-302779-A describes an image bearing member having a surface layer that contains a compound having seven or more carbon atoms with a polymerizable functional group and an aliphatic acid carbon cyclic structure, a binder resin, and a charge transport material. However, although this surface layer is suitable for improving releasability, it has inferior mechanical durability since the resin and the binder resin are not cross-linked. Therefore, this image bearing member does not prolong the working life of the image bearing member. Furthermore, since the layer contains a large amount of polymer, charges are easily blocked where a large amount of polymer is present, which tends to cause a rise in the voltage. It is therefore highly possible that the image quality deteriorates.
JP-H05-216249-A and JP-2005-55589 describe an image bearing member having a cross-linked type charge transport layer formed of a copolymer formed by a radical polymerizable monomer and a radical polymerizable charge transport material. Although this image bearing member has improved abrasion resistance, it has significantly inferior releasability. Therefore, impurities easily adhere to the surface, adversely impacting the life span of the image bearing member.
JP-2001-272802 describes an image bearing member having a photosensitive layer that contains a wax having a long chain alkyl group to have a good combination of the mechanical durability and the releasability. This image bearing member has good releasability at the initial stage but mechanical durability is insufficient.
JP-2001-166510-A describes an image bearing member having a surface layer manufactured by cross-linking a fluorine-containing compound (which is a material for reducing surface energy), a silicon-based compound, and a charge transport material to reduce the surface energy. Although providing good releasability, the cross-linked layer having such a structure has a high permeability to gas, so that the cross-linked layer is easily damaged by gas produced by the charger. It is therefore highly possible that image quality deteriorates once the image bearing member is used for an extended period of time.
Thus, as is clear from the foregoing description, there is a trade-off between mechanical durability and releasability. Therefore, designing an image bearing members striking a poor balance between the two has been inevitable up to now.